The invention relates to nuclear reactor dip seal maintenance systems and more particularly to liquid sodium dip seal maintenance systems of nuclear reactor components having rotatable members.
In nuclear reactor designs, well known in the art, a reactor vessel with fuel assemblies disposed therein and having an inlet and outlet for circulation of a coolant in heat transfer relationship with the fuel assemblies is sealed by a closure head located on top of the reactor vessel. In certain designs, the closure head comprises one or more rotatable plugs. These plugs, which may be of varying sizes disposed eccentrically within each other, serve at least two purposes. One purpose is, of course, to seal the reactor internals inside the reactor vessel. Another purpose is to support refueling machines. The rotation of the rotatable plugs positions the refueling machines in appropriate relationship to the fuel assemblies in the reactor vessel to facilitate the refueling process. Since the rotatable plugs must be able to rotate relative to each other, the plugs are mounted so as to define an annulus between them. This annulus, while allowing the rotation of the plugs, also provides a path for the release of radioactive particles located in the reactor vessel. Accordingly, seals are provided at various locations across the annulus to prevent this release of radioactive particles. These seals may be of various types located at one or more locations along the annulus. As is well known in the art, a typical annulus seal may be a liquid dip seal. Furthermore, in a liquid metal fast breeder reactor, the liquid used in the dip seal may be sodium. The dip seal prevents the migration of radioactive particles, which may be present in a cover gas that fills the void between the top of the reactor coolant and the bottom of the closure head, from the cover gas space into the annulus above the dip seal. In certain applications, the liquid sodium dip seal may be placed at such a location or with heating elements surrounding it so that the sodium remains in a liquid state during reactor operation. In other applications, the sodium in the dip seal is allowed to solidify during reactor operation, thereby, increasing the effectiveness of the seal. However, in any application, the sodium must be a liquid during reactor refueling to enable the plugs to rotate.
After a period of exposure, the cover gas which may be an inert gas such as argon becomes contaminated with not only radioactive particles but also oxides and hydrides of sodium. Because the cover gas is in contact with the sodium dip seal, the sodium therein also becomes contaminated with these compounds. These impurities cause a crust to form on the sodium dip seal free liquid surface. In addition, the amount of sodium within the dip seal is reduced due to the formation of these impurities. These impurities may continue to develop to the extent of resisting rotation of the plugs or even to the extent of preventing this rotation. Therefore, it is necessary to periodically remove the impurities from the dip seals. Several methods and mechanisms are known for so removing these kinds of impurities from components but they have not been effective when used in reactor operation.
One method for removing impurities from sodium which is well known in the art involves the application of steam to the deposits. The steam causes a chemical reaction which causes the impurities to recombine and be forced out of the dip seal. There is, however, a major problem with this method. This problem is that liquid sodium reacts violently in contact with steam or oxygen. To reduce this reaction, the steam is mixed with an inert gas such as argon in such a percentage as to limit this violent reaction. Nevertheless, the use of steam creates the same kind of problems that it seeks to eliminate because the steam itself adds impurities that may combine with sodium elsewhere in the reactor system such as the reactor coolant area or other annuli thereby creating a similar problem at a different location. Therefore, while the steam method may temporarily clean the dip seals, the problem may recur at another location.
The most common method for removal of the crust associated with these impurities is by a contact tool such as a scraper. In this method, a scraper is placed in contact with the crust while the plugs are rotated thereby scraping the crust loose and preventing its accumulation to the extent of preventing rotation of the plugs. However, in this method, the scraping action may accumulate the crusty impurities between the scraper and plug annuli surfaces as to clog the annuli and prevent further rotation of the plugs. Of course, this difficulty prevents this method from being a satisfactory method of removing these impurities.
In U.S. Pat. No. 3,819,478, to Thorel et al, issued June 25, 1974, there is described an apparatus that attempts to provide a pair of properly located redundant dip seals in a top shield plug annulus that prevents liquid sodium vapors or cover gas from entering the annulus below the top frozen dip seal in a rotating plug. The Thorel patent also attempts to describe a means of removing impurities from sodium before the sodium is introduced into the dip seals. While the Thorel patent may describe one method of removing impurities from sodium outside of a dip seal it does not teach a method of removing impurities that may contaminate the sodium in a dip seal that is not easily accessible.
While the prior art contains several methods for cleaning sodium deposits on reactor components and for removing impurities from sodium, it does not teach an effective method or non-contacting tool for removing impurities from relatively inaccessible reactor components, such as liquid sodium dip seals.